1. Field of the Invention
The present invention concerns wire electrodes used in machining metal parts by spark erosion.
2. Description of the Prior Art
In this type of machining, as described in document FR-A-2 418 699 for example, a wire electrode is driven in longitudinal translation and a portion of said wire is guided and stretched along a straight line segment displaced laterally along a path in the vicinity of a metal part to be machined. An electrical generator produces a potential difference between the part to be machined and the metal wire forming the electrode. Machining occurs in the machining area between the wire electrode and the metal part and progressively erodes the part and the wire.
Improvements to the quality of spark erosion electrode wire have been sought for a long time, combining good mechanical resistance to traction, good electrical conductivity of the wire and more regular production of the eroding sparks in the machining area between the electrode wire and the part to be machined.
For example, document U.S. Pat. No. 4,287,404 describes a method and device for making a spark erosion electrode wire having a wire core surrounded by a layer with a low boiling point, such as zinc, cadmium, tin, lead, antimony and bismuth. The outer metal layer is deposited by a step of cold electrolytic deposition from a bath of metal salt in aqueous solution, followed by a drawing step. A method of this kind has the drawback of producing electrode wire in which the outer layer vaporizes too quickly and provides insufficient protection of the core during spark erosion.
It has been found advantageous to heat the wire after electrolytic deposition of the surface layer of metal with a low boiling point, to form a diffused alloy, as taught in document EP-A-0 185 492. However, a process of this kind cannot be used to produce an electrode wire with a surface layer of diffused alloy in which the thickness and the structure are perfectly controlled.
Document U.S. Pat. No. 4,169,426 describes another method of producing a spark erosion electrode wire having a wire core surrounded by a metal layer, in which a conductive wire is passed continuously through a bath of molten metal and then quickly cooled to prevent intermetallic compounds forming at the interface between the wire core and the outer metal layer. The wire is then heat treated for several minutes at 320.degree. C. before it is drawn to the required final diameter. A method of this kind also has the disadvantage that the thickness and the structure of the coating are not totally controlled.
Document CH-A-655 265 proposes preheating the conductive wire by the Joule effect by passing an appropriate electric current through it before it enters a bath of molten metal.
Document EP-A-0 811 701 teaches a similar method of producing spark erosion wire by passing the wire through a bath of molten metal, in which the wire is heated by the Joule effect while passing through the bath of molten metal. This method is no better than the previous ones in terms of controlling the thickness and structure of the coating.
During fabrication of a spark erosion electrode wire by any of the methods mentioned above, it is particularly difficult to adjust the quantity of deposited metal forming the coating, and above all the proportion of diffused metal, and consequently the nature of the alloy phases obtained. For example, in the method of document EP-A-0 185 492, thermal diffusion of the metals is very sensitive to the temperature of the wire and the heating time, and so obtaining a particular structure of the distribution of the metal concentrations in the coating requires very accurate control of the wire heating conditions during the diffusion step, which is difficult to achieve in an industrial process. What is more, it is not possible to obtain a wire structure having a surface layer of pure zinc covering an intermediate diffused brass layer in one simple operation.
The problem is that the coating of the spark erosion electrode wire must be sufficiently thick to constitute the wear layer when the wire is used for spark erosion machining and must have satisfactory properties suited to the machining conditions that will apply. A diffused brass layer having a .beta. phase crystalline structure, favoring a higher speed of spark erosion, is desirable, in particular. However, obtaining the .beta. phase layer is highly dependent on the temperature conditions during the diffusion step, and it is difficult to achieve consistent industrial production that is perfectly controlled.
Document FR-A-2 502 647 concerns a method of brass-plating metal parts such as a long wire for use in tire armatures. A thin (0.28 .mu.) copper layer is deposited on an iron core, after which a zinc layer is applied electrolytically and the wire is heated to assure complete diffusion of the copper and the zinc, forming an .alpha. brass layer encouraging adhesion between the wire and the rubber of the tire. The coating obtained on the wire is too thin to be usable for spark erosion and the .alpha. brass is not suitable for spark erosion. The method employed would not be able to produce wire with a coating of sufficient thickness for spark erosion.
The problem addressed by the present invention is that of designing simple and inexpensive means for significantly enhancing the possibilities and the flexibility of manufacture of spark erosion electrode wires including at least one diffused alloy or metal layer around an electrically conductive wire core. To adapt the structure of the electrode wire to varying requirements it is necessary to be able to vary at will the concentration of metals in the various layers forming the coating and the thickness of the layers.
In an improvement on the invention, the aim is simultaneously to improve the adhesion of the metal or alloy layer to the core, to enable subsequent drawing of the wire without significant deterioration of the outer metal layer.
Another object of the invention is to provide a means of forming a metal or alloy layer of the above kind at will whose surface portion contains a high proportion of a metal with a low boiling point, such as zinc, cadmium, tin, lead, antimony or bismuth.